Re: PWM Controller question

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Sue Gaeta <sgsparky-at-prodigy-dot-net> 

Jim,

Several years ago, a new breed of AMers started to show up on the 75 meter 
ham band with "PDM rigs" that use Mosfets in their designs. They really 
sound great, and I know they aren't modulating them with mod. transformers, 
but to be totally honest here, I haven't done enough research to find out 
exactly what they are doing. If they sound that good, using transformers to 
modulate is not only bulky, and inconvinient, but absurd! The only good 
thing about them is that you get complete isolation, and the circuitry is 
less complex. However, I don't think complexity is an issue for those guys 
that are pioneering this whole SSTC (and it's varients) movement.

I am a follower rather than a leader. Show me a schematic and I will build 
it! Although my friend has built many transmitters with vacuum tubes, and 
is perfectly capable of installing and operating the most modern 
transmitter equipment all the way up to 100KW, he has never delved into the 
inner workings of those solid state transmitters, so I can only get vacuum 
tube help from him!

Sue

Tesla list <tesla-at-pupman-dot-com> wrote:
Original poster: "Jim Lux"


----- Original Message -----
From: "Tesla list"
To:
Sent: Wednesday, June 23, 2004 8:22 PM
Subject: Re: PWM Controller question


 > Original poster: Sue Gaeta
 >
 > ...and that was with a lousy modulation transformer!
 > I showed my project to a friend of mine who is a broadcast engineer, and a
 > high fidelity freak. He is also an amateur radio operator with a home
 > brew high fi AM transmitter. Just for sh**s and grins, he connected his
 > modulator and power supply to my VTTC. The quality of the audio was
 > unbelievable!
 >
 > The mod transformer that I have in there now is out of an old military
 > transmitter which is known for poor quality audio (an ART 13). If I wanted
&g!
t; good fidelity I would have to use a transformer that was 4 times as heavy,
 > and would not fit in my modulator deck! Good high side modulation makes
for
 > a very bulky, and heavy rig, but the results are indeed cool!
 >
 > I have this crazy idea that bigger sparks will give better bass response,
 > so I would love to see Steve Ward give his interrupter a rest, turn down
 > the rail voltage half way, and high side modulate that sucker! People say
 > it can't be done, but it is done with 50, 60, 100, and 120 Hz all the
time.
 > Finding a suitable mod transformer that can handle that much current, and
 > has the correct impedance match would be challanging, and maybe even
 > impossible though. Someone once suggested substituting the mod transformer
 > with an SCR or something. If someone could come up with a way of high side
 > modulation with active power devices in place of a modulation transformer
 > it wou! ld be great! Even if he settled with driver side modulation, I would
 > love to see what kind of audio comes out of a 10 foot arc! The nice thing
 > about high side, high fidelity modulation is that throbbing base will
 > produce long streamers if you are fully modulating (ie: the rail voltage
 > will peak at double the! supply voltage with no modulation, and then dip
to
 > 0 volts during an audio cycle).

Rather than using transformer plate modulation, why not use a DC supply that
is modulated. There are several schemes for doing this, all of which would
be familiar to AM transmitter designers. After all, what you are basically
building is a AM transmitter, no?

There are some nice IGBT based PWM DC supplies that have 20 kHz modulation
bandwidths around. Of course, you quickly find that the design challenges
for the supply are more than for the RF PA. I ran into this when designing
a SSB PA using Class E PA stages. The PA! was easy, the power supply was not
(because essentially, you're trying to make a 3kW hifi audio amplifier) In
any case, going to PWM means that the modulation transformer (that which
boosts the voltage up to match the high load impedance of your plate
modulated amp) gets much smaller.

What about something along the lines of the Harris digital broadcast amps.
Essentially a giant power DAC.


 > Sue
 >
 > Tesla list wrote:
 > Original poster: "Eastern Voltage Research Corporation"
 >
 >
 > > > I think Dan's PlasmaSonic design is good, but I could see some
 > > > improvements in using (possibly) rail modulation. Real high power
 > plasma
 > > > audio modulation will be seriously cool! I have seen it while arcing
 > > > something "semi-grounded" to a AM radio station tower many years past
 > > > (don't ask!). My grid modulated VTTC has been the clo! sest I have seen
 > > > yet, but quality was unreliable.
 >
 > Absolutely. This *is* the way to do it, although high-side modulation is
 > more involved and requires a lot more
 > equipment.
 >
 > Sue Gaeta's high-side audio modulated VTTC was stunning and definitely out
 > performed the PlasmaSonic as far as audio quality goes.
 >
 > Dan
 >
 >
 >
 >